1. Field of the Invention
The present invention relates to a sensor for determining directional variations in a physical characteristic in a moving sheet of material. In particular, the present invention relates to a sensor for determining the orientation of fibers in a moving sheet of fibrous material based upon the directional variation of a physical characteristic, and a method of using the sensor.
2. Description of Related Art
Many of the qualities and characteristics of a fibrous sheet material depend, at least in part, on the orientation of the fibers forming the material. Furthermore, the orientation of the fibers within the sheet material can, to a certain degree, be controlled during the manufacturing process.
For example, paper is typically formed by ejecting a slurry of water and paper pulp fibers from a headbox onto a moving porous belt, frequently called a "wire." The water component of the slurry drips through the wire leaving a mat of paper fibers on the wire. The mat is then subject to additional processing, such as pressing, drying, calendering, coloring, coating or the like to form a finished paper product. Many of the qualities and characteristics of the finished paper product are related to the orientation of the paper pulp fibers in the original mat.
That is, as the slurry is ejected onto the moving wire the paper pulp fibers may have a tendency or probability to become longitudinally oriented in one particular direction. If this tendency is high, a large number of fibers are oriented in one particular direction and the resulting paper product will be highly anisotropic. If the probability or tendency of the fibers to be oriented in any particular direction is low, that is the fibers are truly in a random orientation, the resulting paper product will be isotropic. The properties, such as tensile strength, of anisotropic products may exhibit directional variations while the properties of isotropic products tend to be independent of direction. Depending on the intended application, the optimum degree of anisotropism may vary.
Furthermore, for anisotropic products, the direction of predominant fiber orientation is also important. The directionality of the properties of anisotropic products usually depends on the predominant direction of fiber orientation. That is, if the predominant fiber orientation is at an angle to the machine direction, then the directionally dependant properties will typically exhibit either a maximum or a minimum at the same angle. Therefore, information relating to both the direction of the predominant fiber orientation, or the fiber orientation angle, as well as the degree of anisotropism is useful in evaluating fiber orientation in a fibrous sheet of material. Typically, in papermaking, the fiber orientation angle is defined as the angle between the machine direction, the direction of the moving wire, and the predominant direction of fiber orientation.
The tendency of the paper pulp fibers to have particular orientation depends on a large number of factors, including the geometry of the headbox, the speed of the wire, the speed of the ejected slurry, the direction of the ejection of the slurry relative to the wire, and cross-currents within the slurry. Many of these factors can be controlled. Therefore, it is useful to be able to quickly and accurately measure the degree of anisotropism and the fiber orientation angle during the manufacturing process. This allows for the manufacturing process to be adjusted to achieve the desired product qualities.
One present method of determining fiber orientation involves, measuring and counting the orientation of individuals fibers within the fibrous sheet. Because this method is extremely labor intensive, it is expensive, time consuming, and generally impractical for use in a manufacturing situation.
Another, presently used method of measuring the fiber orientation in a fiber sheet utilizes the relationship between tensile strength and fiber orientation. In this method, the tensile strength of a sample of the fibrous sheet is tested in several directions. A polar plot of the measured tensile strengths typically results in a shape resembling an ellipse. The ratio of the major axis of the ellipse to the minor axis of the ellipse provides an indication of the degree of anisotropism of the fibers in the sheet. Further, the angle between the major axis of the ellipse and the machine direction corresponds to the fiber orientation angle.
Although this test has been found to be relatively reliable, it is destructive to the fibrous sheet, time consuming, and cannot be conducted on-line. Given the speed of modern paper making equipment, this can result in the production of large quantities of substandard product before the test results are available. Furthermore, because the fiber orientation can vary substantially in the cross direction, i.e., across the width of the wire normal to the machine direction, the test must be conducted on samples taken at various intervals, or slices, across the sheet. This further increases the time and expense of this method of determining fiber orientation.
In addition, a number of devices employing optical, sonic, X-ray, or microwave attenuation techniques have been devised to measure fiber orientation. However, such devices are typically expensive, and are not well suited for the harsh environment often found in paper mills.